(1) Field of the Invention
The present invention relates to a power transmission gearbox, and also to an aircraft provided with such a gearbox.
In particular, an aircraft of the rotorcraft type has a rotor contributing to providing the aircraft with at least part of its lift. A helicopter may thus have a rotor that is referred to as its “main” rotor for providing it with lift and with propulsion.
(2) Description of Related Art
In order to set a rotor into rotation, a rotorcraft is provided with a power plant comprising at least one engine.
In addition, a gearbox may be interposed between the engines and a rotor in order to set the rotor in rotation. The gearbox also has the function of driving the rotor at a speed of rotation that is low compared with the high speed of rotation of the engines. By way of illustration, a turboshaft type engine possesses a drive shaft that rotates at a speed of the order of several tens of thousands of revolutions per minute (rpm), while the rotor rotates at a speed lying in the range 250 rpm to 450 rpm, for example.
Thus, a helicopter has at least one engine setting a main gearbox into motion, the main gearbox driving the main rotor in rotation.
In addition, the gearbox may set accessories into motion. Such accessories may include in particular lubrication pumps, hydraulic pumps, compressors, alternators, and fans, for example.
A prior art gearbox presents one mechanical inlet drive system per engine. Each mechanical inlet drive system is thus set into motion by a respective engine.
Furthermore, each mechanical inlet drive system serves to set accessories into motion. The mechanical inlet drive systems also act together to drive a gear that is sometimes referred to as the “coupling” gear.
Conventionally, a gear set comprises two toothed gears that mesh together. The gear having the fewer teeth is usually referred to as the “pinion”.
Various types of gear are known.
A gear known as a “spur teeth” gear or “gear with spur teeth” has a top disk substantially parallel to a bottom disk and an edge face extending in elevation from the bottom disk to the top disk. The teeth are then arranged on the edge face of the gear. A bevel spur teeth gear presents the feature of possessing an edge face that is conical in shape.
A gear referred to as a “face teeth” gear is also known. A face teeth gear is described in particular in Document EP 0 227 152. A face teeth gear has a top face and a bottom face together with an edge face extending in elevation from the bottom face to the top face. Teeth are then arranged on the periphery of at least one of the top and bottom faces, the teeth extending in elevation from that face.
The bottom face may be in the form of a bottom disk, and the top face may be in the form of a top disk parallel to the bottom disk.
The face from which the face teeth extend may also have some other shape, in particular it may be conical in shape. For example, the bottom face may be in the form of a bottom disk, while the top face may be in the form of a truncated cone, the face teeth being formed on the conical top face.
A face teeth gear is also known as a “crown gear”, or a “contrate gear”, or “face gear” or gear with face teeth”
A face gear conventionally meshes with a spur teeth gear that presents a diameter smaller than the diameter of the face gear. The driving member of the gear set as represented by the spur teeth gear thus presents a diameter that is less than the diameter of the driven member of the gear set as represented by the face gear. The face gear is thus used in a gear set having the function of reducing a speed of rotation.
In the above-described prior art gearbox, the coupling gear is a spur teeth gear.
The coupling gear sets at least one speed reduction stage into motion in order to drive a rotor in rotation. Furthermore, the coupling gear may be secured to a shaft for setting lubrication pumps into motion via spur teeth gears.
That teaching is advantageous. Nevertheless, the ever-increasing number of accessories that need to be driven leads to difficulties in designing the power transmission architecture of the gearbox. Specifically, the mechanical inlet drive systems extend longitudinally in order to be connected to accessories. Consequently, gearboxes are tending to present increasing lengths in order to be able to set multiple accessories into motion. Such lengthening of the gearbox tends to reduce its stiffness, and also leads to difficulties of integrating the gearbox on an aircraft.
In addition, each mechanical inlet drive system may have a single shaft driving various accessories. If such a single shaft breaks accidentally, that causes all of the accessories co-operating with that shaft to malfunction.
Furthermore, numerous gear sets are sometimes used for mechanically connecting the accessories to a mechanical inlet drive system, e.g. in order to obtain particular speeds of rotation at the inlet to each accessory. Such numerous gear sets can be heavy, expensive, and difficult to arrange.
Finally, that architecture tends to cause the accessories to be moved away from the gearbox. The casing of each accessory is then not connected directly to the gearbox casing. Under such circumstances, the casing of an accessory can move relative to the gearbox casing. Such relative movement can have harmful consequences on the drive of the accessory by the gearbox because of the stiffness specific to each casing.
In this context, driving accessories by means of a gearbox can lead to difficulties.
The technological background includes Document FR 2 830 307. An inlet pinion driven by a power source engages two face gears. Each face gear is secured to an intermediate pinion, each intermediate pinion meshing with another face gear. That document describes a rocking power transmission gearbox and does not provide teaching dedicated to driving accessories.
Document U.S. Pat. No. 6,612,195 describes a transmission system for dividing inlet mechanical torque into multiple mechanical power transmission paths. Inlet pinions with spur teeth then drive a face gear.
Document EP 0 971 155 describes a system having spur teeth gears driving two face gears on a common axis.
Document US 2002/084120 describes spur teeth gear sets.
Document DE 203 16 481 describes a tractor having gear sets comprising bevel gears and pinions.
Document US 2007/0137342 is also known.